1. Field of the Invention
This invention relates generally to digitized alpha numeric displays and more particularly to a means for reducing instability in the readout of a binary digital signal subject to errors from noise, jitter, or threshold uncertainty superposed thereon.
2. Description of the Prior Art
The present invention is adapted for use with an on-board aircraft radar antenna system. The system includes a mechanical drive for redirecting the antenna to a desired position.
In this application, the drive system is controlled by a digital circuit which includes a potentiometer having a control knob located at a control panel in the aircraft cockpit. Rotation of the control knob generates a digital command signal which causes the drive system to rotate the antenna to a position dependent on the amplitude and direction of rotation of the control knob. A digital readout indicator located in the cockpit is coupled to receive the digital command signal which reflects the antenna position as the control knob is rotated.
Preferably, on the completion of rotation of the control knob, the sequential progression of numerals on the readout indicator will come to rest at a finite value, thereby indicating the present position of the antenna. However, in practice, due to noise, jitter, or threshold uncertainty signal components superposed on the analog position signal, the progression of numerals on the readout indicator cotinues to fluctuate, that is, move erratically from one number to another, leading to uncertainty as to the actual position of the antenna.
Noise may be defined as a random or periodic disturbance superposed upon the useful signal tending to obscure its information content. Such noise may be produced, for example, by thermal agitation in electronic circuit components or by coupling from adjacent circuits, such as switching power supplies and other pulse sources. Jitter refers to the small, rapid aberrations in the indicated signal which are time, amplitude, frequency, or phase related, and which may result, for example, from the wiping action of a movable contact on the control knob. Threshold uncertainty results from the condition where the control knob is left in a transition position such that is at the threshold of producing a change in digital values, and hence the digital readout does not have a unique input signal.
The prior art has attempted to solve the problem of unstable indication by such techniques as high-grade smoothing filters, or dedigitization of the digital signal with positive feedback. While these approaches have to some extent mitigated the problems associated with noise or jitter or threshold digital uncertainty, none have solved the problems associated with the occurrence of all three conditions simultaneously.
High-grade smoothing refers to the use of a filter circuit, which may comprise a network of resistors and capacitors which are coupled to the analog signal to be processed, thereby removing selectively a broad band of undesired frequency components which contribute to noise and jitter. However, this technique can result in a noticable delay between rotation of the control knob and the resultant display due to phase shift in the network, and does not prevent the signal from being adjusted close to a digital threshold which will cause uncertainty.
The use of dedigitizing and positive feedback is a technique in which an analog signal from the position control is converted to digital form by means of an analog/digital convertor for transmission. The digitized signal is then reconverted to analog form by means of a digital/analog converter. A small fraction of the recovered analog signal is summed with the analog signal from the position control to provide feedback to the analog/digital converter. This approach is effective in reducing threshold uncertainty, but is limited to less than one bit of authority, since positive feedback may permit circuit oscillation.
A further technique that has been suggested is the elimination of low order bits, which effectively disables the system response to small changes. However, this results in an undesirable loss of sensitivity to the operator, since the readout appears to progress in quantum steps such as 2.0, 2.3, 2.6,. etc., rather than the smaller increments with which the control knob may be capable of being adjusted and which the operator will expect to observe.